This research proposal seeks to answer fundamental questions on the contractile machinery of the bladder, specifically to determine the functional relevance of SM1 and SM2 myosin isoforms (that differ in the C-terminus). We have shown previously that alternative splicing of a single SMHC gene both at the amino terminus (SM-A and SM-B) and carboxyl-terminus (SM1 and SM2) generate four different SMHC isoforms: SM1A, SM1B, SM2A and SM2B. Urinary bladder smooth muscle expresses predominantly SM1B and SM2B myosin isoforms. Our laboratory recently demonstrated that the NH2-terminal isoform SM-B isoform is an important determinant of the kinetics of urinary bladder smooth muscle contraction. However, the functional relevance of C-terminal isoforms SM1 and SM2 has not been completely understood. During bladder development SM1 myosin is expressed early in fetal smooth muscle, whereas SM2 myosin expression late in fetal development coincides with smooth muscle differentiation and organ development. In addition to developmental regulation, SM2 myosin expression is altered during ageing and in obstructive bladder disease, thus the ratio of SM2:SM1 is subject to change. However the exact relevance of SM2:SM1 ratio to smooth muscle pathophysiology is yet to be understood. To understand the functional significance of the SM2 myosin, we have recently generated a mouse model that is deficient in SM2 myosin but expresses SM1. SM2 -/- mice die postnatally between 15-30 days due to bladder obstruction and loss of SM2 was not compensated by an increase in SM1 level. On the other hand SM2 heterozygous (-/+) lived to term but ~50% of male mice developed severe obstructive bladder disease by 15 months, suggesting when the SM2 levels and/or the SM2:SM1 ratio is further altered it can predispose to bladder dysfunction. Our hypothesis is that maintaining a tissue specific SM2: SM1 ratio in the bladder body and urethra is critical for coordinated function and voiding and when this ratio is perturbed, it could modify contractility .Based on the preliminary data, we additionally hypothesize that a switch in SM2:SM1 ratio will affect myosin filament assembly and contribute to bladder dysfunction. Therefore, the major goals of the current proposal are to study how loss of SM2 myosin affects 1) urinary bladder smooth muscle development and maturation in male and female mice, 2) myosin filament structure and distribution within the bladder body and urethra and, 3) contractility of the bladder and urethra and 4) the molecular mechanism for bladder dysfunction , specifically why the male SM2 Het mice develop bladder dysfunction at Old age. These studies are a major step towards defining the roles of C-terminal isoforms SM1 and SM2 in bladder smooth muscle physiology and will allow us to understand how a switch SM1/SM2 ratio could contribute to pathophysiology of the bladder smooth muscle.